The invention relates to a system and method for controlling an internal combustion engine coupled to an emission control device.
In direct injection spark ignition engines, the engine operates at or near wide-open throttle during stratified air-fuel ratio operation in which the combustion chambers contain stratified layers of different air-fuel ratio mixtures. Strata closest to the spark plug contain a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, a three way catalyst optimized for NOx storage, known as a NOx trap or catalyst, is typically coupled downstream of the first three-way catalytic onverter.
During lean, rich, and stoichiometric operation, sulfur contained in the fuel can become trapped in the emission control device in the form of SOx. This gradually degrades emission control device capacity for storing NOx, as well as emission control device efficiency. To counteract sulfur effects, various sulfur decontamination methods are available.
One method for determining when to perform a decontamination cycle uses an estimation scheme. In this approach, an amount of SOx stored in the NOx trap is estimated based on operating conditions. For example, an amount of SOx stored is estimated based on driving distance. In another example, the amount of stored SOx is estimated based engine operating conditions. Then, when the estimated amount of stored SOx reaches a predetermined value, the decontamination cycle is performed. Such a method is described in U.S. Pat. No. 5,657,625.
The inventors herein have recognized a disadvantage with the above approach. In particular, depending on the setting of the predetermined value, fuel economy will be significantly affected. For example, if the value is set too high, decontamination cycles will be too frequent. With too frequent decontamination, fuel economy will be degraded since fuel is too often spent to perform decontamination. Similarly, with too infrequent decontamination, fuel economy will be degraded since fill and purge cycles may be inefficient.
An object of the invention claimed herein is to provide a method for determining a penalty of performing emission control device decontamination cycles.
The above object is achieved, and disadvantages of prior approaches overcome, by a method of determining a penalty of a decontamination cycle, the decontamination cycle for an emission control device coupled to an internal combustion engine, the method comprising: calculating an excess quantity of fuel used during any decontamination cycle; and generating the penalty based on said excess quantity of fuel.
By determining the penalty for performing decontamination cycles, it is possible to more accurately determine whether decontamination should be performed. In other words, the penalty can be used to improve vehicle performance by optimizing engine performance for given operating conditions. Also, decisions can be made as to whether other engine operating conditions may more efficiently meet emissions and fuel economy targets than those which require decontamination.
An advantage of the above aspect of the present invention is improved fuel economy without degrading emission performance.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.